My tuppence-worth, having stayed in two, and visited a third, a couple of months ago:

JW KL is slightly cheaper/older/rougher around the edges, but I found the service more friendly/natural and it has an amazing pool. There is more local beach life - while it is remote, there are enough reasonably priced beach-front restaurants, bars and massage places if you get bored of the resort and don't have (or want to use) wheels

JW P is a mega-resort, the most child-friendly, the most restaurants and activities, plus a resort-style shopping centre just outside - families love it; if you think children should be seen and not heard (or, indeed, not even seen...), then avoid

Ren P is the newest, a bit more City glamorous, a bit more boutique, style over substance. The staff try hard, and nothing is too much trouble. Smallest pool (by far), more adult, less to do, and a bit more remote - there are 2 restaurants in the resort and one massage hut on the beach, and that's about it. Nice gym though!

So, it depends what you like - others, I'm sure, will disagree with my assessment. I guess KL is more "real" Thailand, if that's what you want, but a good 90 minute drive from Phuket. The JW P and Ren are in Mai Khao, which is a semi-gated community near the airport - a poor man's Nusa Dua - I didn't think the beach was as nice as KL (or Nusa Dua, for that matter). I quite enjoyed doing a few days at JW KL then moving to the Ren. Oh, and the beach massages and beers are cheaper in KL